Documentation: add initial benchmarks

Documentation/benchmarks/README.md

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+# Benchmarks
+
+etcd benchmarks will be published regularly and tracked for each release below:
+
+- [etcd v2.1.0](etcd-2-1-0-benchmarks.md)

Documentation/benchmarks/etcd-2-1-0-benchmarks.md

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+## Physical machines
+
+GCE n1-highcpu-2 machine type
+
+- 1x dedicated local SSD mounted under /var/lib/etcd
+- 1x dedicated slow disk for the OS
+- 1.8 GB memory
+- 2x CPUs
+- etcd version 2.1.0
+
+## etcd Cluster
+
+3 etcd members, each runs on a single machine
+
+## Testing
+
+Bootstrap another machine and use benchmark tool to send requests to etcd cluster.
+
+## Performance
+
+### reading one single key
+
+| key size in bytes | number of clients | target etcd server | read QPS | 90th Percentile Latency (ms) |
+|-------------------|-------------------|--------------------|----------|---------------|
+| 64                | 1                 | leader only        | 1534     | 0.7        |
+| 64                | 64                | leader only        | 10125    | 9.1      |
+| 64                | 256               | leader only        | 13892    | 27.1      |
+| 256               | 1                 | leader only        | 1530     | 0.8       |
+| 256               | 64                | leader only        | 10106    | 10.1      |
+| 256               | 256               | leader only        | 14667    | 27.0      |
+| 64                | 64                | all servers        | 24200    | 3.9      |
+| 64                | 256               | all servers        | 33300    | 11.8      |
+| 256               | 64                | all servers        | 24800    | 3.9      |
+| 256               | 256               | all servers        | 33000    | 11.5      |
+
+### writing one single key
+
+| key size in bytes | number of clients | target etcd server | write QPS | 90th Percentile Latency (ms) |
+|-------------------|-------------------|--------------------|-----------|---------------|
+| 64                | 1                 | leader only        | 60        | 21.4 |
+| 64                | 64                | leader only        | 1742      | 46.8 |
+| 64                | 256               | leader only        | 3982      | 90.5 |
+| 256               | 1                 | leader only        | 58        | 20.3 |
+| 256               | 64                | leader only        | 1770      | 47.8 |
+| 256               | 256               | leader only        | 4157      | 105.3 |
+| 64                | 64                | all servers        | 1028      | 123.4 |
+| 64                | 256               | all servers        | 3260      | 123.8 |
+| 256               | 64                | all servers        | 1033      | 121.5 |
+| 256               | 256               | all servers        | 3061      | 119.3 |